Conical scan antenna



June 23, 1953 J. J. BRADY CONICAL SCAN ANTENNA Filed'sept. 18, 1945 F'IG.I

PULSE-ECHO" SYSTEM WITH AUTOMATIC TRACKING INTENSITY INVENTOR JAMES J. BRADY ATTORNEY Patented June 23, 1953 CONICAL SCAN ANTENNA James J. Brady, Belmont, Mass, assignor, by mesne assignments, to the United States of America, as represented by the Secretary of the Navy Application September 18, 1945, Serial No. 617,139

4 Claims.

This invention relates to radio pulse echo systems having conical scanning, and more particularly to such systems arranged for automatic tracking.

The primary object of the invention is to generally improve systems of the described character.

In conical scanning it has heretofore been the practice to use an antenna system including a parabolic or equivalent reflector such that the radiation pattern at any instant is substantially symmetrical. With automatic tracking the apparatus seeks to keep the target at the intersection or crater formed by the rotation of the main radiation lobe. Unfortunately, there are additional points of intersection between the sec ondary lobes at each side of the main lobes, and if the target is positioned near one of these secondary intersections the apparatus may track the target at that intersection, thus causing a very false aim.

The object of the present invention is to overcome this difiiculty, which is done by making the radiation pattern such as to eliminate intersections of secondary lobes. More specifically, I employ an antenna system having an asymmetrical radiation pattern such that the secondary radiation at one side of the main lobe is appreciably greater than the secondary radiation at the other side of the main lobe.

One comparatively simple and convenient way to accomplish the desired result is to modify a parabolic reflector in the same way as is done to provide a so called cosecant square antenna. Such an antenna has been used for airborne search equipment of the plan-position-indicator or so-called PPI type, but has not been used as part of a conical scan system. In terms of structure, in the present invention such an antenna is spun about an axis extending generally in the direction of (but at a slight angle to) the axis of the main lobe.

To accomplish the foregoing general objects and other more specific objects which will hereafter appear, the invention resides in the method and apparatus elements and their relation one to another as are hereinafter more particularly described in the following specification. The specification is accompanied by drawings in which:

Fig. 1 schematically represents an automatic tracking pulse-echo system utilizing conical scan;

- Fig. 2 illustrates one antenna arrangement for producing a conical scan;

Fig. 3 represents an alternate antenna arrangement for producing a conical scan;

Fig. 4 shows the radiation pattern obtained with a conventional antenna system; and

Fig. 5 shows the radiation pattern obtained by means or the present invention.

Referring to the drawing, and more particularly to Fig. 1, electromagnetic energy is radiated by an appropriate antenna system i2, and is reflected and concentrated by a suitable reflector it. Heretofore the reflector system has most commonly been a paraboloid. The axis i S of the system is mounted for orientation in various directions so that it may be aimed at a target. This aiming or tracking of the target is preferably accomplished automatically by suitable apparatus, diagrammatically represented by the rectangle I8. One system for this purpose is that disclosed in the Patent 2,473,175, issued to Louis N. Ridenour on June 14, 1949, and entitled Radio Direction-Finding System. In Fig. 1 it will be observed that the axis of propagation 20 of the radio beam is disposed at a slight angle to the axis of rotation it, thus producing what is commonly termed a conical scan.

Referring now to Fig. 4, the solid line radiation pattern and the dotted line radiation pa tern represent the propagation in positions of rotation apart. When the antenna system is properly aimed, the target will be located on the scan axis 2 i, or at the intersection or crater 22. It will be understood that the receiver 58 (Fig. 1) is so designed as to produce error voltages of proper sense and magnitude when the target is disposed on either side of the crater 22, and these error voltages are used to automatically correct the aim of the antenna system so as to tend to bring the target back to the crater 22 in the radiation pattern.

In Fig. 4 the angle between the axis of propagation or main lobe and the axis of rotation or scan 2! is indicated by the dimension d. The manner in which this angle is obtained is not necessarily a feature of the present invention. One method suggested in the patent of Louis N. Ridenour previously referred to, is to tilt the reflector at the desired angle relative to the axis of rotation. In said patent it is also explained that the reflector may be rotated without rotating the antenna if it be desired to maintain one plane of polarization, and that the antenna and reflector may be bodily rotated together if it be desired to rotate the plane of polarization.

However, it is not necessary to tilt the reflector, and in Fig. 2 there is shown an antenna system including dipole 24 fed by a coaxial feed line 26 having a reflector disc 28 and carrying a reflector 30, only a small portion of which is shown. The feed line 26 is provided with a choke 32 which is axially adjustable, and by appropriate adjustment of the position of the choke the axis of propagation 34 will be tilted at an angle to the axis of rotation.

In Fig. 3 a modified system is shown in which the antenna is displaced from the focus of the reflector. Thus the antenna may be located at 36, and is displaced from the focal point 38 of the reflector 40. This will shift the axis of propagation from the axis of rotation 42 to the angular position 44.

In all of these known systems as so far described the radiation pattern includes secondary lobes, and is substantially symmetrical relative to the main lobe. Thus in Fig. 4 the main lobe 46 is accompanied by the secondary lobes 48, 48 and 50, 50'. Similarly the main lobe 52 is accompanied by secondary lobes 54, 54' and 56, 56.

Difiiculty has arisen in automatic tracking systems because of ambiguity or false tracking. It has been found that this is caused by intersections or cross-over points of the secondary lobes of radiation. Thus if a target should appear at the point 60 (Fig. 4), the system will tend to lock on the target and will track the target at that intersection. This will produce a substantial error corresponding to the angle 0. In the case of the intersection at 6B the signals occur in proper polarity to tend to aim the intersection at the target.

If the target should appear at an intersection such as 62 the polarities are reversed, and the point is a point of instability. At such a point if the target is to the right, or in the main lobe, the system will tend to correct the error until the target is properly located in the crater 22; but if on the other hand the target appears even slightly to the left of the point 62, the system will move the target to the point 60.

Similar conditions exist at the other cross-over points in the radiation pattern.

In accordance with the present invention the difliculty is overcome by eliminating false crossover points such as the points 60 and 62. This is best done by producing a radiation pattern which is asymmetrical relative to the main lobe. Thus referring to Fig. 5, the solid line pattern comprises a main lobe l and a secondary radiation 12 on the right hand side which is appreciably stronger than the secondary radiation 14 on the left hand side of the main lobe. When the antenna system has turned through 180 the situation is reversed and the secondary radiation 12' on the left hand side of the main lobe is substantially greater than the secondary radiation 14 on the right hand side of the main lobe 10'. It will be seen that there are no secondary crossover points, for the radiation '!2 lies above the radiation 14, and similarly the radiation 12 lies above the radiation 14. The only cross-over point is the main crater 16 within the main lobes, and therefore the ambiguity heretofore resulting from secondary cross-over points is wholly eliminated.

There are various ways in which the asymmetrical radiation pattern may be obtained. One of the simplest and that which results in the radi ation patterns shown in Fig. 5 is to use a known modification of a parabolic reflector commonly termed a cosecant square antenna. This is indicated in Fig. l in which either a barrel shaped insert or a half paraboloid insert 80 is secured to Wei 4 one side 82 of the parabolic reflector [4, with a quarter wave spacing at 84. Such antennas have heretofore been used in airborne P. P. I. systems for scanning, but in such system the antenna was rotated about a substantially vertical aXis as viewed in Fig. 1, instead of being spun about a substantially horizontal axis.

In Fig. 2 the reflector insert i shown at 3|. The reflector and insert must be rotated with the feed line and dipole. The insert is preferably disposed diametrically opposite the direction of deviation of the propagation axis 34.

In Fig. 3 the reflector insert is shown at 4|, and here again it will be understood that the reflector and insert are rotated with the feed line and the displaced dipole 36. The insert 4| is preferably disposed on the side towards which the antenna is displaced, so that the tendency of the insert to shift the main lobe will be consistent with the shift produced by the off-set antenna.

One interesting feature of the present invention is that the reflector may itself be used as the means to produce the desired angle between the main direction of propagation and the axis of rotation. Appropriate gearing, terminating in gear 85 (Fig. 1), may be used to rotate the reflector. In such case there is no need for the axially adjustable choke shown in Fig. 2, or the off-set antenna location shown in Fig. 3, because the modification of the reflector may itself be made to throw the radiation pattern to one side of the axis of rotation.

Such an arrangement is convenient because it is unnecessary to rotate the feed line and dipole. Inasmuch as it is necessary in all forms of my invention to rotate the reflector, it becomes convenient to rotate the reflector alone. As another form of my invention, the tilted reflector arrangement suggested in the patent of Louis N. Ridenour referred to above may be used, except that the usual parabolic reflector is modified by an insert which will cause the radiation pattern to be asymmetrical relative to the main lobe.

It will be understood that for the present purpose the antenna need not be exactly of the cosecant square type because an approximation will do, and even a complete departure from the cosecant square type may be used provided only that the radiation on either side of the main lobe is so asymmetrical that no secondary cross-over points are obtained. A broad or widely spread main lobe may be used for the same purpose, but

this lessens the directivity and efficiency of the system as a whole.

In referring to automatic tracking it will be understood that the invention is useful not only for systems in which the antenna is mounted on gimbals or the like for rotation in azimuth and in elevation relative to the apparatus supporting the same, but also for systems in which the antenna spins on an axis fixedly mounted on the apparatus, but in which the position of the entire apparatus may be altered. For example, a selfguided glider or bomb may be provided with a conical scanning system, and the error voltages developed in the receiver for automatic tracking may be fed to appropriate rudders and elevators, or to appropriate guide vanes, which will continually correct the direction of travel of the glider or bomb in order to point it toward the target. In either case (gimbals or fixed axis) the antenna system is aimed at the target, and in either case it is important to eliminate any ambiguity which would result in false aim. However, the invention is of' particular importance with self-guided missiles because a small reflector is used for practical reasons, and with a small reflector the side lobes are of relatively large magnitude. If the missile is not released at the proper angle it may readily lock on the target with an error of 30 or more. This possibility is eliminated by the present invention.

It is believed that the construction and operation of the present invention, as well as the advantages thereof, will be apparent from the foregoing detailed description. It will also be apparent that while I have shown and described my invention in preferred forms, changes may be made in the structures disclosed without departing from the spirit of the invention as sought to be defined in the following claims.

I claim:

1. In a radio pulse-echo system an automatic target tracking antenna comprising, a radiator and a reflector, said reflector comprising a parabola and a half parabola disposed substantially between said parabola and said radiator, said half parabola being in contact with said parabola along the edge of said half parabola passing through the axis and being spaced therefrom by a continuously increasing amount to a point substantially at the midpoint of the other edge of said half parabola,the spacingbetween said parabola and said half parabola at said midpoint of said other side being substantially a quarter wave length at the frequency of operation of said antenna, and means for rotating said reflector and said radiator about the axis of said parabola to provide a radiation pattern having a single cross-over point.

2. In an automatic tracking system, an antenna comprising a reflector formed of two similar half parabolic surfaces meeting in a plane, the axes of said half parabolic surfaces intersecting said plane and each other at acute angles, and a feed system for said reflector cooperating therewith to produce an asymmetrical beam of radiation having a directive axis disposed at an acute angle to an orientation axis with the energy concentrated to the side of said beam furthest removed from said orientation axis, and means for rotating said reflector and feed system together about said orientation axis to produce a conical scan wherein said beam overlaps itself throughout the rotation thereof, the asymmetry of said beam as aforesaid insuring a single cross-over point of said beam.

3. In an automatic tracking system, an antenna comprising a parabolic reflector and a feed system arranged to produce a directive beam having an axis of directivity disposed at an acute angle to an orientation axis, said beam having a main lobe and secondary side lobes of radiation, means for rotating said reflector and feed system together about said orientation axis to produce a conical scan, and an auxiliary reflector having the shape of a half parabola attached to said parabolic reflector and spaced therefrom and arranged to concentrate the secondary side lobes of said beam to the side thereof furthest removed from said orientation axis whereby said beam has a single crossover point within said main lobe during the rotation thereof.

4. In an automatic tracking system, an antenna comprising a parabolic reflector and a feed system arranged to produce a directive beam of radiation having an axis of directivity disposed at an acute angle to an orientation axis, said beam having a main lobe and secondary side lobes of radiation, an auxiliary reflector having the shape of a half parabola attached to said parabolic reflector and arranged to concentrate the secondary side lobes of said beam to the side thereof furthest removed from said orientation axis, and means for rotating said antenna about said orientation axis whereby the directivity axis of said beam traces the surface of a cone and said beam overlaps itself during rotation thereof, the concentration of the secondary side lobes as aforesaid insuring a single point of crossover within the main lobe of said beam during rotation.

JAMES J. BRADY.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain July 20, 1936 Number Number 

